1. Field of the Invention
The present invention relates to ceramic positive temperature coefficient resistors and somewhat more particularly to a method of stabilizing the hot resistance of such resistors.
2. Prior Art
German Pat. No. 1,490,713 discloses ceramic positive temperature coefficient resistors having non-blocking heat applied contact coatings thereon. It is suggested therein that contact coatings be provided by applying a thin film of indium or indium-gallium onto a surface of the ceramic body, then applying a heatable silver preparation onto the thin film and then heating the resultant structure in an oxidizing atmosphere to adhere, burn-in or sinter the contact coatings onto the ceramic body.
However, it has been discovered that with certain specific types of positive temperature coefficient resistor materials, the hot resistance of resistors composed of such materials attains only about 0.3 to 3% of the value such resistors exhibit when just the indium or indium-gallium film was applied. As used herein, the term "hot resistance" will be understood to refer to the maximum resistance of a positive temperature coefficient resistor above the Curie temperature thereof.
A reduction of the hot resistance of a positive temperature coefficient resistor decreases the high increase in resistance required at temperatures above the Curie temperature of the resistor. Accordingly, it would be highly desirable to avoid a reduction in the hot resistance of a resistor.